Adhesives used in dental work are subject to exacting physical property requirements. At the same time, such adhesives, due to the likelihood of contact with the gums and other tissue, must employ non-toxic and non-irritating components.
In restorative dentistry, for example the preparation of crowns, bridges, and dentures, a casting operation is used which requires a reasonable degree of accuracy in recreating the natural contours of teeth and gums. For such purposes, a female casting, or "impression" is made in the mouth, using a curable dental "impression" material.
In preparing such impressions, an impression tray is used. This impression tray is a U-shaped trough which fits over the teeth and gums, and which may be made of plastic, metal or other materials. A curable polymer impression material is placed in the tray, and the tray pushed down (or up) onto the teeth and gums. Following cure, the tray and cured impression material is removed.
The impression material itself must be non-toxic and non-irritating, and must rapidly cure without the generation of excessive heat. The impression material must also exhibit low shrinkage in order to retain accuracy, and must be elastomeric in order that it may be removed from the teeth and gums without destroying the mold thus produced.
Because of these relatively stringent requirements, two component addition-curable, room temperature vulcanizable (RTV-2) organopolysiloxane elastomers are widely used as dental impression materials. These curable systems generally include hydrogen-functional organopolysiloxanes and unsaturated organopolysiloxanes, with a platinum hydrosilylation catalyst contained in one component. The two components are mixed just prior to use, and cure rapidly. However, in order to remove the cast impression from the oral cavity, it is necessary for the impression material to exhibit greater adhesion to the dental tray than to the teeth and gums. In general, impression materials have insufficient adhesion for this purpose, and thus a dental adhesive is used between the tray and impression material. However, the nature of the impression tray surface on the one hand, and the impression material on the other hand, makes formulation of an adhesive which can suitably adhere to both tray and impression material difficult.
In U.S. Pat. No. 4,763,791, which may be referred to for background information, a kit is disclosed which contains dental trays as well as impression material and dental adhesive. However, the identity of the adhesive and its composition is not disclosed. U.S. Pat. No. 5,108,286 attests to the difficulties encountered due to the stringent requirements for adhesion of impression material to mouth tissues and the impression tray. Rather than employ a separately applied adhesive, the '286 inventors chose instead to employ a special bi-adhesive laminate, containing a high strength innermost plastic film coated on both surfaces with an adhesive layer. Removable foils are employed to keep the adhesives fresh. A benefit of the '286 process is that two different adhesives may be used, one tailored for adhesion to the impression tray, and one tailored for adhesion to the impression material. However, the use of such products is cumbersome and requires some dexterity in conforming the adhesive laminate to the impression tray.
It would be desirable to provide a dental adhesive which is formulated to have the requisite adhesion to both the impression tray and to the impression material without requiring two separate adhesive compositions, or the use of adhesive laminates. It would be further desirable to provide such an adhesive in a liquid form which can be brushed or sprayed onto the impression tray. It would be further desirable to provide an adhesive which can be applied to an impression tray several hours before use without relinquishing its desirable adhesive properties.